Portable emergency breathing devices typically include a cylinder of compressed breathing grade air having a cylinder pressure of approximately 20.68 MPa (3000 psi) and a pressure demand supplied air respirator. One or more pressure regulators are required to reduce the unregulated pressure in the cylinder down to some target respirable pressure, such as about 249 Pa (1 inch of water). Where a first and second pressure regulator are utilized, the second stage pressure regulator may be worn by the operator as part of a respirator device.
First stage pressure regulators generally utilize a piston that is allowed to translate axially for engagement and disengagement with a valve seat. The valve seat is typically interposed between the cylinder and the pressure regulator. At one end of the piston is a piston seat frequently made of a compliant material such as polycarbonate or brass that is acted upon by the unregulated pressure. The other end of the piston generally has a surface area which is acted upon by the regulated pressure, so as to force the piston seat against the valve seat to shut off the flow of air when the regulated pressure is achieved. A spring is typically included to bias the piston seat toward the valve seat. Air flows from the cylinder until the pressure on the regulated side of the piston is high enough to generate an adequate force against the piston surface to cause the piston seat to move the piston seat against the valve seat, thereby stopping the flow of air from the cylinder.
Some designs of first stage regulators utilize a rotating knob or other threaded member to force the piston against the valve seat and shut-off the flow of compressed air. The amount of force generated between the piston seat and the valve seat depends on how far the knob is closed. Deformation of the valve seat can occur in some circumstances.
As a result of manufacturing limitations and machining tolerances, the piston, the piston seat, and the valve seats are generally not perfectly aligned axially. Additionally, irregularities in the machining of the valve seat may become imprinted on the piston seat. In the event the regulator is disassembled and reassembled for maintenance, any indentations or deformations on the piston seat may not be precisely realigned with the valve seat. Although the spring force and force on the piston generated by the regulated and unregulated air pressure are known, the force required to form a seal is somewhat unpredictable. As a result of surface imperfections on the piston and valve seats and/or a mismatch between the piston seat and the valve seat, the force required to deform the piston seat and form a seal and shut-off of the flow from the cylinder may be greater than is generated by the target regulated pressure acting on the regulated side of the piston. Consequently, the valve remains open until the regulated pressure is sufficient to form a seal and shut-off the flow of air. The increased force necessary to form a seal can correspond to a regulated pressure and/or a variability of the regulated pressure outside the target regulated pressure range.